The overall object of this project is to examine the effects of microwave hyperthermia on tumor and normal tissue microphysiology and attempt to relate the information obtained to pertinent radiobiological phenomena. In the first year our object was to obtain initial parameters in the C3H mouse-MCA tumor system. Distributions in tissue oxygen levels, pH, and blood flow have been determined using ultramicroelectrodes of tip dimensions of approximately 1 micron. Determinations have been made in mammary adenocarcinoma transplanted in C3H mice under both normal and perturbed conditions. Under normal conditions, widely differing values of all three parameters are found in a single tumor. Complete histograms detailing the results obtained in many tumors have been obtained. Tumor tissue responses to microwave induced hyperthermia and x-irradiation have been measured. Hyperthermia was induced by using microwaves at a frequency of 2450 MHz. Heat was monitored using thermocouples. It was found that tissue pO2 rose until a temperature of 40-41 degrees Centigrade was reached. Above this, there is a breakdown of blood flow and a decrease in tissue pO2. Tumor tissue was found to be acidotic. Most of the measured values were in the range of 6.4 to 7.1 pH units, the mean was approximately 6.7. Within different areas of the same tumor, distinct heterogeneities in the pH distribution were found to exist. Very low pH values (5.8-6.3) were observed in large, ulcerated tumors. Upon heating for one hour at a temperature of 43 degrees Centigrade the tissue pH fell 0.5-1.0 pH unit. Histograms have again been obtained. Also measurements performed 72 hours after x-irradiation showed a slight shift to higher oxygen values. The implications of these results will be examined further.